Why does air way collapse when pressure outside exceeds pressure inside?

[sameeralord]

Ok this makes sense but pressure inside an airway is determined by hydrostatic pressure and when air moves fast hydrostaic pressure is decreased and converted to kinetic energy. So this collapses the airway. My question is doesn't kinetic energy of moving molecules exert a pressure on the walls of the airways. Why is it that only hyrdostatic pressure determines the pressure inside an airway? Thanks!

 

[Andy Resnick]

Pressure always acts normal to the vessel walls, the fluid velocity (shear stress) is tangent to the walls.

 

[sameeralord]

Pressure always acts normal to the vessel walls, the fluid velocity (shear stress) is tangent to the walls.

Hey thanks for the answer. I like it but can you give me a link or bit more description how fluid velocity acts tangent to the walls. Oh wait do you simply mean this

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So only hydrostatic pressure exerts pressure on the walls! Is it that simple? but total pressure is both hydrostatic and velocity right. Since the pressure that exerts on the walls is what matters the airway collapses. If this is right Thanks a lot for you! and I got it!